Many applications have utilized the condensational growth of airborne particles to enable their measurement or collection. For ultrafine particles the initiation of this growth requires the creation of a region of vapor supersaturation. U.S. Pat. No. 6,712,881 describes a method to create this supersaturation in a laminar flow, wherein a flow passes through a wet-walled tube, or other container, the second portion of which is warmer than the first portion. U.S. patent application Ser. No. 13/218,393 describes a different method in which the temperature profile of the wetted walls has a cold section, a warm section, and a final cold section. These laminar flow systems that create water vapor supersaturation have come to be known as condensation growth tubes
These condensation growth tubes work by exposing an air flow to wetted surfaces at different temperatures. The wetted surface is commonly implemented as a wick, which is a porous medium that will readily draw water into its pores via capillary action. During operation of these condensation systems, water evaporates from the warm portion of the wick, and thus the wick must be continually supplied with more water. To accomplish this, various wetting methods have been used. In some systems, water pumped to the outside of a cylindrical wick is drawn to the inside by capillary action. The excess water cascades down the length of the tube via gravity. Other systems use an internal reservoir wetting system, wherein the bottom end of the wick is submerged in a reservoir of water.
Disadvantages of this prior art are the complications in the water handling, risk of flooding and sensitivity to being tipped. Protrusions into the flow channel, as required for the reservoir wetting system, are a problem for some applications.
FIG. 1 shows one prior art system for passive wetting of a wick 100. A dike 150 (sometimes called the “standpipe”) having a base 155 is a necessary barrier to prevent water 140 from seeping under or through the wick 100 where it would drip into lower parts of the device. Water 140 is transported up the wick 100 by capillary action. An aluminum housing 110 provides structure and a good thermal connection to the heater or cooler 120. In the case of a wick that consumes water, a filling system 130 is required, comprised of a water level sensor controlling a pump or valve, and an air vent 125. The air vent 125 communicates with the inlet flow to provide pressure equalization between the head space of the water reservoir and the flow. The system ensures the reservoir 145 neither fills above the upper edge of the dike 150, nor goes completely empty. In the case of a cold wick where water vapor condenses, the system must discard water.